1. Field of the Invention
The present invention relates to a method for modifying a polymer surface and a method for producing a polymer product. In particular, the present invention relates to a method for modifying a polymer surface and a method for producing a polymer product based on the use of a supercritical fluid.
2. Description of the Related Art
In recent years, various processes have been suggested, in which a resin is formed and processed by using a supercritical fluid which has permeability like a gas and which functions as a solvent like a liquid. For example, Japanese Patent Application Laid-open No. 10-128783 suggests the following method. That is, the supercritical fluid is permeated into a thermoplastic resin, and thus the supercritical fluid functions as a plasticizer to successfully lower the viscosity of the resin. Therefore, this action of the supercritical fluid is utilized to improve the fluidity and the transfer performance of the resin upon the injection molding.
Various methods have been also suggested to realize highly advanced functions or high performance, in which the function of the supercritical fluid as the solvent is utilized to improve, for example, the wettability of the polymer surface. For example, Japanese Patent Application Laid-open No. 2001-226874 discloses the fact that the fiber surface can be made hydrophilic by dissolving polyalkyl glycol in the supercritical fluid to make contact with the fiber. Japanese Patent Application Laid-open No. 2002-129464 discloses a batch process for realizing a highly advanced function of a polymer surface, in which a polymer is allowed to make contact with a supercritical fluid in which a solute as a functional material has been previously dissolved, in a supercritical state, i.e., at a high pressure to perform a dyeing.
However, as a result of diligent investigations performed by the present inventors, it has been revealed that the following problems arise in the method for modifying the polymer surface in which the supercritical fluid is used as the solvent as described above.
At first, the polymer is swelled such that the supercritical fluid, which functions as the plasticizer, is allowed to make contact with the entire polymer surface. Therefore, when the thickness of the polymer is thick, the interior of the polymer tends to be foamed. Further, when the supercritical fluid makes contact with the polymer, the glass transition temperature (Tg) of the polymer is lowered. Therefore, the polymer is deformed, and it is difficult to maintain the shape accuracy. For this reason, it is difficult that the method for modifying the polymer surface as described above is applied to polymers other than a polymer such as a film or a fiber in which the thickness is thin and the cross-sectional area is small. Therefore, the way of use has been limited. When a polymer having a thick thickness is used, it is possible to suppress the foaming phenomenon in the polymer and the sudden decrease in the glass transition temperature of the polymer by making the pressure-reducing speed for the supercritical fluid to be extremely slow. However, the throughput is deteriorated. Therefore, the merit of the use of this method is lost.
The solubility as a solvent of the supercritical fluid, especially of carbon dioxide in the supercritical state is equivalent to that of n-hexane. For example, even when an entrainer is used as an auxiliary, it is not approved that the solubility of a solute in the supercritical fluid is sufficient. In other words, the ratio of the permeation of the functional agent as the solute into the polymer is smaller than the ratio of the permeation of the supercritical fluid such as the supercritical carbon dioxide into the polymer. Therefore, the concentration of the solute permeated into the polymer is extremely low. For this reason, it has been necessary that the supercritical fluid containing the solute is allowed to make contact with the polymer for at least 10 to 30 minutes.
Further, there is a strong correlation between the pressure of the supercritical fluid and the solubility of the solute. When the vessel, which is allowed to have a high pressure by filling with the supercritical fluid, is released or opened, then the pressure in the vessel is suddenly reduced in a batch process for modifying the polymer surface, and the solubility of the solute is extremely lowered. Therefore, the solute is consequently deposited during the reduction of the pressure. When the vessel is allowed to have a high pressure again, the internal pressure of the vessel is restored in a relatively short period of time, while the solute, which has been once deposited, is not necessarily dissolved immediately in the polymer again. Accordingly, a problem arises, for example, such that the piping of the apparatus to be used for the batch process is clogged with the deposited solute. In this case, it is difficult to recover the solute in a powdery state in order to use the solute as the functional agent again, in which the cost is expensive as well. Further, it is necessary to provide a mixing tank for uniformly or homogeneously dissolving the solute in the supercritical fluid at a predetermined concentration. The apparatus itself is expensive.
Further, the conventional batch process is a process for modifying the entire polymer surface. Until now, there has been no report on any process in which a part of a polymer surface is modified selectively and finely.